The solution which shows maximum freezing point must have minimum number of solute particles.
(1) $\left[ Co \left( H _{2} O \right)_{6}\right] Cl _{3} \rightarrow\left[ Co \left( H _{2} O \right)_{6}\right]^{3+}+3 Cl ^{-}, i =4$
(2) $\left[ Co \left( H _{2} O \right)_{5} Cl \right] Cl _{2} \cdot H _{2} O \rightarrow\left[ Co \left( H _{2} O \right)_{5} Cl \right]^{2+}+2 Cl ^{-},i=3$
(3) $\left[ Co \left( H _{2} O \right)_{4} Cl _{2}\right] Cl \cdot 2 H _{2} O \rightarrow\left[ Co \left( H _{2} O \right)_{4} Cl _{2}\right]^{+}+ Cl,i=2$
(4) $\left[ Co \left( H _{2} O \right)_{3} Cl _{3}\right] \cdot 3 H _{2} O \rightarrow\left[ Co \left( H _{2} O \right)_{3} Cl _{3}\right], i =1$
So, solution of $1$ molal $\left[ Co \left( H _{2} O \right)_{3} Cl _{3}\right] \cdot 3 H _{2} O$ will have minimum number of particles in aqueous state.